Chip-on-board (COB) and surface-mount (SM) devices are types of integrated circuits (IC) well known in the art. A COB is generally an IC chip attached with an adhesive to a substrate, such as a circuit board, with wires electrically interconnecting the chip to conductors on the substrate. The wires and the interconnection are typically formed using wire bonding techniques, in which very thin electrically-conductive wires, often on the order of about 0.25 millimeter or less in diameter, are bonded to leads on the chip and to bond pads on the substrate. Suitable wire bonds can be achieved with various techniques, including thermosonic and ultrasonic bonding. In contrast, SM IC devices are generally characterized by being electrically and mechanically attached to circuit board with one or more terminals or leads that are soldered, such as with a tin-lead solder, to a corresponding number of conductors on the surface of the substrate. A prominent example of a SM IC is a flip chip, which has bead-like terminals typically in the form of solder bumps near an edge of the chip. After registering a flip chip to its corresponding conductor pattern on a substrate, heating above the melting or liquidus temperature of the solder produces solder joints that serve to both secure the chip to the substrate and electrically interconnect the flip chip circuitry to the conductor pattern.
While wire-bonded devices are very robust, their wires are susceptible to damage during handling. Therefore, COBs are typically encapsulated in order to protect their wires. However, suitable encapsulant materials introduce a mismatch in coefficients of thermal expansion with the chip and substrate, which lowers the resistant of a COB to thermal cycle fatigue. As a result, COBs are widely use in consumer electronic products having modest temperature requirements (e.g., up to about 80.degree. C.), but have found limited use in more demanding environments, such as automotive and aerospace applications requiring an upper temperature capability of 125.degree. C. or more. Where both COBs and SM devices have been used, their different attachment methods considerably complicate the processing of the circuit board assembly. Specifically, the prior art has found it necessary to attach and process either the SM devices or the COB devices first, and then perform a second process for the remaining devices. For example, if SM devices are mounted first, solder paste is printed on the circuit board, and the SM devices are then mounted and reflow soldered to the board using a thermal cycle of about four to about seven minutes, during which a peak temperature of at least about 210.degree. C. is attained.
Thereafter, the COB devices are mounted by depositing a silver epoxy adhesive on the circuit board, placing the COB device on the adhesive, curing the adhesive, wire bonding the device, and then encapsulating the device to protect the wires. The adhesive cure is a critical step in mounting COB devices, in that a proper cure is necessary to yield a strong and durable attachment that will withstand thermal cycling without fracture or delamination from the device or substrate. Preferred silver epoxies require an oven cure during which a temperature of typically about 130.degree. C. to about 150.degree. C. is sustained for about thirty to about sixty minutes.
If so desired or required by other process considerations, the COB devices can be attached first by performing the COB attachment process before performing the SM attachment process. With either approach, it can be seen that combining COB and SM devices on a single circuit board is encumbered by the requirement that each type of device is attached at a different time during the assembly process. The different thermal treatment requirements for the adhesives and solders suitable for mounting COB and SM devices, respectively, have prevented any further simplification of the assembly process.